1. Field of the Invention
The present disclosure relates to a liquid crystal display (LCD) device, and particularly, to an LCD device having a column spacer for maintaining a cell gap and a push preventing column spacer, and a method for fabricating the same.
2. Discussion of the Related Art
Demands for various display devices have increased with development of an information society. Accordingly, many efforts have been made to research and develop various flat display devices such as Liquid Crystal Display (LCD) devices, Plasma Display Panels (PDPs), Organic Electro Luminescent Displays (OLEDs) and Vacuum Fluorescent Displays (VFDs). Some of the flat display devices have already been applied to displays for various equipments.
Among the various flat display devices, LCD devices have been most widely used due to their advantageous characteristics such as thin profile, lightweight and low power consumption, and thus are substituting for Cathode Ray Tubes (CRTs). In addition to mobile type LCD devices such as LCD devices for notebook computers, LCD devices have been developed for computer monitors and televisions to receive display broadcasting signals.
In order to use LCD devices in various fields as a general display, LCD devices should offer a high picture quality, such as a high resolution and high luminance with a large-sized screen, while still maintaining such characteristics as lightweight, thin profile and low power consumption.
The LCD device may be categorized into an LC injecting type and an LC dispensing type according to a fabrication method thereof.
In case of the LC dispensing type of LCD device, an LC is dropped on one substrate and the substrate is bonded to another substrate. When using a ball spacer, the ball spacer is rolled on the substrate together with the LC. This may cause a difficulty in maintaining a proper cell gap.
Accordingly, a column spacer fixed to a predetermined part on a substrate has been proposed. In the LC dispensing type of LCD device, a column spacer is formed on a color filter substrate, and an LC is dropped onto a TFT substrate. Then, the two substrates are bonded to each other to form a panel.
However, the LC dispensing type of LCD device may have the following problems. Although not shown, the column spacer contacts upper and lower substrates with large contact areas, and a large frictional force is applied between the column spacer and the lower substrate. Accordingly, once the column spacer has moved to a touching direction, the upper substrate may not be restored to the original state. In this case, a space between the lower and upper substrates having no column spacer has a cell gap larger than that of a part where the column spacer is installed. On a region touched by a user's finger or a pen, a liquid crystal is scattered. And, a liquid crystal is not normally driven on the touched region and the peripheral region due to a deficient or excessive state of the LC. This may cause a touch defect, i.e., stains may occur at an interface between the touched region and the peripheral region.
The reason why the touch defect occurs on the LCD device having a column spacer is because the column spacer is fixed to one substrate, and comes in contact with an upper surface of another substrate. That is, the touch defect results from a wide contact area between the column spacer and the substrate.
In order to solve the touch defect, has been proposed an LCD device having a gap maintaining column spacer and a push preventing column spacer.
The conventional LCD device having a gap maintaining column spacer and a push preventing column spacer will be explained in more detail with reference to FIGS. 1 to 3.
FIG. 1 is a planar view of a liquid crystal display (LCD) device including a double column spacer in accordance with the conventional art, FIG. 2 is an enlargement sectional view taken along line ‘II-II’ in FIG. 1, which shows a column spacer contacting a circular protrusion arranged on a gate line, and FIG. 3 is an enlargement sectional view taken along line ‘III-III’ in FIG. 1, which shows a column spacer and a push preventing column spacer contacting circular protrusions arranged on gate lines.
As shown in FIGS. 1 to 3, the conventional LCD device comprises a lower substrate 11 and an upper substrate 31 facing each other, and an LC layer 41 filled in a space between the lower substrate 11 and the upper substrate 31.
On the lower substrate 11, gate lines 13 and data lines 19 for defining pixel regions are formed to cross each other. Thin film transistors (T) are formed at intersections between the gate lines 13 and the data lines 19. And, a pixel electrode 27 is formed at each pixel region.
The thin film transistor (T) includes a gate electrode 13a extending from the gate line 13, a semiconductor layer 17a formed to cover the gate electrode 13a, a source electrode 19a extending from the data line 19 and formed at one side of the semiconductor layer 17a, and a drain electrode 19b spacing from the source electrode 19a by a predetermined interval and formed at another side of the semiconductor layer 17a. 
As shown in FIGS. 2 and 3, a circular protrusion 21 is formed on the gate line 13. The circular protrusion 21 consists of a metal layer pattern 19c of the data line 19, and a semiconductor layer 17a disposed below the metal layer pattern 19c. The metal layer pattern 19c has an additional contact surface by a tail of the semiconductor layer 17a. This may cause changes of PPM (a ratio of a contact area with respect to a total area).
The gate line 13 is formed on the lower substrate 11 so as to insulate metallic lines from each other. A gate insulating layer 15 is formed on the entire surface of the substrate including the gate line 13. And, a protection film 23 is formed on the gate insulating layer 15. Here, the protrusion 21 insulates the gate line 13 and the data line 19 from each other by having the gate insulating layer 15 therebelow.
On the upper substrate 31 facing the lower substrate 11, formed are a black matrix layer 33 for shielding non-pixel regions (e.g., the gate lines, the data lines and TFTs) rather than the pixel regions, a color filter layer 35 having R, G and B colors at the respective pixel regions, and a common electrode 37 formed on the entire surface of the upper substrate 31 including the color filter layer 35.
Above the common electrode 37, formed are a first column spacer 39a for maintaining a cell gap, and a second column spacer 39b spacing from the lower substrate 11 by a predetermined interval.
The first and second column spacers 39a and 39b have the same height, and are formed on the upper substrate 31. The first column spacer 39a is arranged at a position corresponding to the circular protrusion 21, and the second column spacer 39b is arranged above the gate line 13 or the data line 19 at a position not corresponding to the circular protrusion 21.
When bonding the lower substrate 11 and the upper substrate 31 to each other, the first column spacer 39a comes in contact with the circular protrusion 21 by a pressure generated during the bonding process. And, the second column spacer 39b is spacing from the protection film 23, the uppermost layer of the lower substrate 11.
As mentioned above, the conventional LCD device has the following problems.
In the conventional LCD device, the column spacer is implemented by disposing the circular protrusion on the gate line. Here, wet and dry etching processes are performed in every direction of the circular protrusion. This may degrade CD uniformity inside a glass substrate (mother substrate). More concretely, some panels inside a glass substrate may not have gap defect owing to an optimized contact area ratio, whereas other panels may have gap defect due to a reduced contact area ratio.
Furthermore, the conventional circular protrusion undergoes exposing and etching processed in every direction. This may cause large CD (critical dimension) changes of the circular protrusion according to a glass substrate (mother substrate) and processing dimensions. Since the circular protrusion has an additional contact area with the column spacer due to a tail of the semiconductor layer in every direction, PPM is greatly changed.